The little details of your descriptions of each mare's experiences make this sooo hot

Rarity swirled the cum and savored its taste on her tongue for the last time, then opened her mouth ever so slightly and prepared to strengthen their sisterly bond with one big sloppy kiss.

Awww~!

Now that was a hot rutting!

I haven't read this chapter yet, but it occurs to me that Dew Drop looks very feminine. In face, it I hadn't read the previous chapters, I'd swear that Dew Drop was female.

And now, I read.

Here, have some corrections:

letting out a sigh she’d been holding in her breast for too long.

That word didn't make any sense in this context. Perhaps if this were an anthro story, it would make sense, but considering how far non-anthro pony teats are from a pony's lungs, I'd say either remove that word or replace it with "chest" or somthing.

Sweetie’s eyes shut tightly and a foolish smile stretched on her face.

Did you mean foalish? Nothing "foolish" seemed to happen here, but Sweetie is still a filly, so foalish would make sense.

Her pussy was burning and, despite all of her lady training, she couldn’t endure that feeling any second longer.

It should be either "a second longer", or "any longer".